140 THE GASTROPODA 
Euthyneura and in the majority of Streptoneura. Even in those 
Gastropods that are hatched out in the adult form, the veliger stage 
can generally be recognised, in a more or less reduced condition, 
within the egg membranes, e.g. in Buccinwm, Cenia, the basommato- 
phorous Pulmonates. In all Gastropods the velum is reduced in 
proportion as the foot develops; nevertheless, in a considerable 
number of pelagic larvae the veliger stage is preserved for a long 
time, and the velum persists, and often develops excessively long 
lobes even after the creeping foot is fully and normally developed : 
such is the case in “ Macgillivraya” (Fig. 121), “ Agadina,” “ Chele- 
tropis,” “ Sinusigera,” “Echinospira,” etc., all of which are special 
pelagic larval forms of Streptoneura which were long considered to 
Fig. 122. 
Shell of a young 
Fig. 121. Purpura —haemastoma, 
““ Macgillivraya,” pelagic Jarva of a_siphonate enlarged, dorsal aspect. 
Streptoneurous Gastropod (Doliwm), ventral aspect, ca, canal of the adult ; 
x 12. f, foot; m, mouth; sh, shell; st, siphon ; fe, e.s, limit of the embry- 
tentacle and eye; ve, lobes of the velum. (After onic shell; sp, spire. 
MacDonald.) (After Dautzenberg.) 
be distinct genera. The velar lobes may even produce lobate 
expansions of the margin of the aperture of the shell, but these dis- 
appear when the velum is absorbed and the shell assumes the adult 
form (Fig. 122). 
IV. DEFINITION. 
The asymmetry of some of the principal organs of the body is 
the chief characteristic of the Gastropoda. The essential feature of 
this asymmetry is that the anus generally lies to one side of the 
median plane; that the ctenidium, the osphradium, the hypo- 
branchial gland, and the auricle of the heart are azygos, or at least 
are more developed on one side of the body than the other; and 
that there is only one genital orifice, which hes on the same side of 
the body as the anus. In other words, one-half—generally the 
morphologically left but topographically right half—of the anal 
complex is either atrophied or has disappeared altogether. This 
asymmetry, expressed by the transfer of the morphologically right 
